1. Field of the Invention
The present invention relates to powder conveying apparatus, and is useful in particular in connection with the conveyance of powder paint from a container e.g. to a spray gun for discharge into an electrostatic field or to an electrostatic spray gun.
2. Description of the Prior Art
In U.S. Pat. No. 3,863,808, issued Feb. 4, 1975 to Charles R. Vertue, there is disclosed a particulate material conveying apparatus in which a flow of compressed gas travelling along a gas flow passage is employed to draw particulate material into the gas flow passage through an inlet passage opening into one side of the gas flow passage. A vibrator is provided for vibrating the inlet passage to counteract clogging of the inlet end of the inlet passage. A gas supply pipe feeds the compressed gas to the vibrator for operating the latter and the vibrator has a compressed air outlet communicating with the gas flow passage for discharging the compressed air from the vibrator to the gas flow. Thus, only one compressed gas supply pipe is required for operating the vibrator and also for entraining the powder.
In the practice of electrostatic spray coating it is desirable and, indeed, sometimes necessary to be able to adjust the rate of throughput of powder paint through the electrostatic spray coating apparatus, e.g. to adjust the apparatus to workpieces of different sizes having different areas to be electrostatically spray coated and to adapt the apparatus to different rates of throughput of the workpieces.
Using the particulate material conveying apparatus disclosed in the aforementioned United States Patent 3,863,808, the rate of throughput of the powder paint was adjusted by varying the rate of flow of the compressed air through the gas supply pipe. However, since the powder inlet passage was relatively narrow, the rate of flow of powder through the powder inlet passage was correspondingly restricted and the variation of this rate of flow which could be achieved by adjusting the compressed gas supply through the gas supply pipe was also correspondingly restricted.
Also, for safety reasons, restrictions have been placed on the magnitudes of the voltages which can be used for electrostatically charging powder paints during spraying operations, and the premissible voltages are incapable of charging powder flowing through spray guns at high rates of flow. The abovementioned prior art particulate material conveying apparatus only allows low rates of powder to be sprayed by using low rates of flow of compressed gas for entraining the powder, which causes a risk of insufficient gas movement to avoid blockage and poor powder feeding resulting from the inability of the low gas flow to properly entrain the powder. Modification of this prior art apparatus to adapt it to low rates of powder flow causes the apparatus to be incapable of handling high rates of powder flow, which are sometimes desired.